Lyocell Method and Device Involving the Control of the Metal Ion Content

ABSTRACT

The invention relates to a method and a device for producing Lyocell fibers, which are extruded from a spinning mass containing water, cellulose and tertiary amine oxide. The spinning mass is obtained from cellulose in a number of process steps, wherein a treatment medium is added to the cellulose, cellulose suspension and/or cellulose solution. In order to be able to implement a stable and environmentally compatible spinning method irrespective of the type of cellulose used, according to the invention, provision is made that the content of at least one type of metal ion destabilising the cellulose suspension and/or cellulose solution is monitored in the cellulose, cellulose suspension and/or cellulose solution and adjusted below a stability limit. With the device according to the invention, the metal ion content is measured via sensors ( 23, 23′ ) and the metal ion content is adjusted using a control device ( 17 ).

The invention relates to a method for producing a Lyocell fibre from a spinning mass which is produced by adding a treatment medium to a cellulose, to a cellulose suspension and/or a cellulose solution.

The invention also relates to a device for the production of Lyocell fibers with a mixing device in which a cellulose, a cellulose suspension and/or a cellulose solution can be charged with a treatment medium.

Methods and devices of this kind are known from the Lyocell technology. With the Lyocell technology threads, fibers, films and membranes are extruded as endless molded bodies from the spinning mass containing cellulose, water and tertiary amine oxide. On account of its environmental friendliness, the Lyocell technology is increasingly replacing the conventional viscose methods. The environmental friendliness of the Lyocell method stems from the solution of the cellulose without derivatisation in an organic, aqueous solvent. From this cellulose solution endless molded bodies, for example fibers and film, are then extruded. Through the extrusion of the molded bodies and the orientation and regeneration of the cellulose occurring in the course of the extrusion, molded bodies of high strength are obtained with numerous possible uses in the textile and non-textile sectors. The name Lyocell was issued by the BISFA (International Bureau for the Standardisation of Man-made Fibers). In the state of the art the Lyocell method is now well documented.

Consequently, tertiary amine oxides are known as solvents for cellulose from U.S. Pat. No. 2,179,181 which solvents can dissolve cellulose without derivatisation. From these solutions, the cellulose molded bodies solvents can be obtained by precipitation.

The processing of the cellulose, dissolved in an aqueous amine oxide, particularly N-methylmorpholine-N-oxide (NMMNO), is however problematical with regard to safety, because the degree of polymerisation of the cellulose decreases when dissolving the cellulose in NMMNO. In addition, amine oxides generally exhibit only limited thermal stability, particularly in the system NMMNO/cellulose/water, and have a tendency to spontaneous exothermic reaction. To overcome these problems and to be able to manufacture Lyocell fibers economically, there is a series of approaches for a solution in the state of the art.

In U.S. Pat. No. 4,144,080, it is stated that at high temperatures the cellulose dissolves more quickly in a tertiary amine-N-oxide and forms a more homogeneous solution if the cellulose is milled together with the preferred ingredients of tertiary amine-N-oxide and water. In WO-94/28219, a method for the production of a cellulose solution is described in which milled cellulose and an amine oxide solution are added in a horizontal, cylinder shaped mixing chamber. The mixing chamber exhibits around its longitudinal axis rotating, axially spaced stirring elements. Apart from NMMNO, N-methylpiperidine-N-oxide, N-methylpyrrolidone oxide, dimethylcyclohexylamine oxide and others can be used as the amine oxide. Mixing in the mixing chamber occurs between 65° C. and 85° C. According to WO-A-98/005702, the cellulose is mixed with the aqueous solution of the tertiary amine oxide in a device, whereby the mixing device exhibits a mixing tool and a container which rotates during mixing.

In WO-A-98/005702, the mixing tool is improved such that it is formed as a paddle, rail or helix and during mixing preferably prevents the formation of deposits on the inner surface of the container. In WO-A-96/33934, a buffer device is described, which comprises a mixing vessel and a conveyor worm as a discharging device. In this way, a continuous production of the cellulose solution is facilitated despite the cellulose being fed in batches.

The method of WO-A-96/33934 has now been further advanced by the method of WO-96/33221, in which a homogeneous cellulose suspension is produced from milled cellulose and an aqueous amine oxide solution in one single step. For this purpose, the pulverised cellulose is brought into contact with the liquid, aqueous tertiary amine oxide and a first mixture is formed. The first mixture is spread in layers on a surface and transported under intensive mixing over this surface. This process can be carried out continuously. Other methods in which the cellulose solution is treated in the form of a thin layer are also known from EP-A-0356419, DE-A-2011493 and WO-A-94/06530.

Also the milling of the cellulose itself is an object of the patent publications. For example, U.S. Pat. No. 4,416,698 mentions it as an advantage if the cellulose is milled to a particle size of less than 0.5 mm. In WO-A-95/11261, preliminarily disintegrated cellulose is introduced into an aqueous solution of a tertiary amine oxide to produce a first suspension. This first suspension is milled subsequently and then converted into a formable cellulose solution with the application of heat and a reduced pressure. In order to feed back the dust, arising from milling or pulverising the cellulose, into the process, filters are used in WO-A-94/28215 through which the cellulose dust is separated from the air. In WO-A-96/38625, a system is described which can disintegrate both cellulose bales as well as cellulose in leaf form. An ejection chute is provided which opens into a device for predisintegrating the cellulose.

In EP-B-0818469 it is suggested that cellulose is dispersed in aqueous amine oxide solutions and the dispersion thus obtained treated with xylanases.

Apart from these efforts to economically produce a homogeneous cellulose solution capable of being spun, there are also attempts to overcome the problem of the decomposition phenomena of the cellulose solution which occurs spontaneously under an exothermic reaction. In Buijtenhuis et al., The Degradation and Stabilisation of Cellulose Dissolved in NMMNO, in: Papier 40 (1986) 12, 615-618 test results are described, according to which metals appear to reduce the decomposition temperatures of the NMMNO in the cellulose solution. Primarily, iron and copper appear to speed up the decomposition of NMMNO. Other metals, such as for example nickel or chrome, also exert a negative influence on the decomposition properties of the cellulose solution in appropriate deposits and appropriate concentration, if they are present in appropriate concentrations. However, in WO-A-94/28210, stainless steel is still used as the material for a spinning head in order to withstand the high pressures during the extrusion of the cellulose solution.

In DE-T-699 13 117, a different route is followed to obtain stable cellulose solutions during the production of Lyocell fibers. The method In this publication does not intervene in the cellulose processing according to the Lyocell method, but rather during the production of celluloses for the Lyocell method. According to the method of this publication, the content of metal ions is reduced to obtain celluloses with a low portion of transition metals, in particular with low portions of iron and copper. Certainly, such celluloses are particularly suitable for cellulose solutions serving as the basis for the Lyocell method, because they reduce the risk of decomposition of the NMMNO in the spinning mass during cellulose processing and spinning in the Lyocell method. However, with the method according to DE-T-699 13 117 it is still problematical that celluloses other than those described in the said publication nevertheless lead to a destabilisation of the cellulose solution and/or cellulose suspension during fibre production and a safe Lyocell method without the destabilisation of the cellulose solution, therefore, can only be ensured with the exclusive use of the celluloses of DE-T-699 13 117.

In addition, during the cellulose processing, the system NMMNO/cellulose/water in the highly concentrated NMMNO region has the property of releasing metal ions from the process apparatus, such as lines, filters, and pumps, which reduces the system stability. In WO-A-96/27035, a method for the production of cellulose molded bodies is described in which at least some of the materials in contact with the cellulose solution contain at least 90% of an element from the group of titanium, zirconium, chrome and nickel down to a depth of at least 0.5 μm. The important aspect with regard to WO-A-96/27035 is that the rest of the composition of the apparatus and piping, where it comes into contact with the cellulose solution, does not contain any copper, molybdenum, tungsten or cobalt. According to WO-A-96/27035, this measure should prevent exothermic decomposition reactions.

Finally, in DE-C-198 37 210, which is taken as the closest state of the art, a homogeneous cellulose solution is produced irrespective of the water content of the cellulose used. In contrast to the current method, here, the cellulose is first transported in the absence of NMMNO under homogenisation in a pulper through an initial shear zone and is only then added to a low water-content NMMNO.

Another way of producing the cellulose solution is followed in DE-A-44 39 149 which forms the closest state of the art. According to the method of DE-A-44 39 149, the cellulose is pretreated enzymatically. To increase the effectiveness of the enzymatic pretreatment, the cellulose can be disintegrated under shearing in water before the pretreatment. Then the pretreated cellulose is separated from the liquor and the separated cellulose is introduced into a melt of NMMNO and water. Here practicably, the separated liquor can be fed back to the pretreatment after supplementing the water and enzyme losses. However, in practice, this type of process management has proven to be impracticable, because the cellulose solution obtained in this way is unstable.

Despite these various approaches to obtaining a homogeneous and stable cellulose solution and to convey it with the avoidance of exothermic decomposition reactions through to the extrusion openings, the environmentally friendly and economical production of a homogeneous cellulose solution and its stability remain problematical.

The object of the invention is therefore to improve the known methods and devices of the Lyocell technology such that, with the highest level of environmental compatibility, the method can be carried out independent of the type of cellulose used in a stable manner and with consistent quality.

This object is solved for the aforementioned method in that the content of at least one type of destabilising metal ions in the cellulose, in the cellulose suspension and/or in the cellulose solution is monitored and adjusted below a stability limit.

For the aforementioned device this object is solved according to the invention in that a sensor, by which the content of at least one type of destabilising metal ion can be acquired in the cellulose, cellulose suspension and/or cellulose solution and a control signal representing the content of at least one type of destabilising metal ion can be output, and a control device are provided through which the metal ion content of the cellulose, cellulose suspension and/or cellulose solution can be adjusted to below a stability limit in dependence of the control signal.

The solution according to the invention is simple and enables any type of cellulose to be used for the spinning mass during the cellulose processing used in the production of Lyocell fibers, irrespective of its content of metal ions destabilising the cellulose suspension and/or cellulose solution, so that it is no longer necessary to carefully select the cellulose according to its content of destabilising metal ions, in particular iron (Fe³⁺) and copper (Cu²⁺) ions before its processing, or to mix several types of cellulose. The production of Lyocell fibers according to the invention eliminates the problem of the cellulose processing in that only certain cellulose compositions can be used, which are specially suitable for the Lyocell process and correspondingly expensive, such as, for example, the celluloses produced according to the method of DE-T-699 13 117. Moreover, any celluloses can be used in the course of the Lyocell method, in particular also celluloses with a high to very high content of destabilising metal ions, which until now could not or could only with expensive additional measures be processed, because they have led to an instable cellulose suspension and/or cellulose solution with the risk of an exothermic reaction. The main source of the metal ions which destabilize the cellulose suspension and/or the cellulose solution appears to be the cellulose itself.

The method according to the invention and the device according to the invention can both be used with the Lyocell method, in which the cellulose is directly pulped in an aqueous solution of a tertiary amine oxide or in a tertiary amine oxide and cellulose solution is produced from it, and also with methods in which initially a cellulose suspension is produced containing essentially water and cellulose and only then a tertiary amine oxide or an aqueous solution therefrom is added to form a cellulose solution.

The stability limit for the metal ion content is determined in dependence of the composition of the cellulose suspension and/or cellulose solution, their temperature and their residence time in the plant from the pulper through to the extrusion of the Lyocell fibers. Each type of metal ion can exhibit different high stability limits. The less a certain type of metal ion destabilizes the cellulose suspension and/or cellulose solution, the higher the stability limit can be. Thus, the stability limit, for example, for copper ions may lie below the stability limit for iron ions due to their stronger destabilising effect. The stability limit can be found experimentally in that for samples of cellulose solutions with different metal ion contents, different temperatures and different times of exposure, the percentage of samples producing exothermic reactions is found. This percentage figure is then used as the probability value for the occurrence of an exothermic reaction. The stability limit can be defined based on a probability value of an exothermic reaction which is realistic for the plant operation. For example, such a probability value may be below 0.01% or below 1×10⁻⁶% so that with the set stability limit an exothermic reaction is only to be expected with a probability of 0.01% or 1×10⁻⁶%.

The method according to the invention and the device according to the invention can be further improved in a series of advantageous embodiments which can be combined with one another as required.

Thus, in an especially advantageous embodiment, the quantity of the treatment medium added to the cellulose, cellulose suspension and/or cellulose solution is adjusted in dependence of the content of the at least one type of metal ion. In this way, the content of the at least one type of metal ion introduced by the cellulose in the cellulose suspension and/or cellulose solution is suppressed below the stability limit. A particularly high level of environmental compatibility and economy of the method can be achieved in that, in a further development, the added treatment medium is recovered in a following processing step in the Lyocell method.

For example, the water added to a cellulose suspension in the course of the pretreatment of the cellulose can be returned as press water from the expressing stage of the cellulose suspension and be reused when pulping the cellulose. In further embodiments, this press water can be purified before it is added to the cellulose and in particular it can be freed at least partially of metal ions.

With the direct pulping of the cellulose in amine oxide or with the addition of amine oxide to the expressed cellulose suspension, recovered amine oxide can be used, which for example can be regenerated from a spinning bath through which the freshly extruded spinning threads are passed and in which the cellulose precipitates. Also, the recirculated tertiary amine oxide can be purified and in particular be freed at least partially of the metal ions it contains before it is added to the cellulose, cellulose suspension and/or cellulose solution. A commercially available ion exchanger can be used to remove the metal ions.

A reduction in the content of destabilising metal ions below the stability limit, which is particularly simple to implement, arises when, in the step in which the recycled treatment medium is added, a fresh, non-recycled treatment medium of the same kind is added to the cellulose, cellulose suspension and/or cellulose solution. For example, fresh water, in particular fully desalinated or partially desalinated fresh water can be fed into the recirculated press water and/or fresh tertiary amine oxide can be fed into the recycled tertiary amine oxide. Since fewer destabilising metal ions are dissolved in the fresh treatment medium due to the mixing in the cellulose, cellulose suspension and/or cellulose solution, the content of destabilising metal ions does not rise too strongly and in particular not above the stability limit despite the return of the treatment medium. In addition, through this method, losses of treatment medium, which are primarily due to the treatment medium remaining in the spinning threads, can be prevented.

If, according to a further embodiment, part of the treatment medium is discharged from the production process and is therefore no longer available for return within the process, then, destabilising metal ions are also removed from the process with the drained off treatment medium. Thus, the content of the destabilising metal ions in the cellulose suspension and/or cellulose solution can also be controlled via the quantity of the discharge of treatment medium which can no longer be returned to the process. The discharged treatment medium is preferably replaced by fresh treatment medium. The quantity of discharged treatment medium depends preferably on the content of destabilising metal ions in the cellulose, cellulose suspension and/or cellulose solution so that as much of the treatment medium as possible can be recycled.

The metal ion content can be particularly effectively controlled if the respective proportions of recycled and fresh treatment medium are adjusted in dependence of the content of the at least one type of metal ion. If, for example, the content of the metal ions increases, then the proportion of the recycled and returned treatment medium and thus the introduction of metal ions into the cellulose suspension and/or cellulose solution is reduced. If the metal ion content decreases, then the proportion of the recycled treatment medium can be increased with respect to the proportion of the fresh treatment medium.

The metal ions can be determined by inline sensors, i.e. sensors, which are arranged in the plant volume through which the cellulose suspension and/or cellulose solution flows during the Lyocell production, and/or in an automatic laboratory analysis device after a manual sampling. In the first case, the signal from the sensors can be used by a control device for the automatic dosing of the relative proportion of the recovered treatment medium and the fresh treatment medium. This type of automatic dosing can, for example, be implemented by valves operated by actuators. With manual sampling, the metal ion content can either be passed to a control device automatically by the automatic laboratory analysis device or the content of the metal ions can be entered manually using an input device.

For the acquisition of metal ions, mass spectrometers, devices for the measurement of atomic absorption, sensors based on Raman scattering and devices with graphite tube technology can be employed.

Due to the method according to the invention and the device according to the invention and their various embodiments, which are independent of one another, it is now possible to process any type of cellulose irrespective of its content. The metal ion content is in each case adjusted below the stability limit. The stability limit is preferably below 10 mg/kg for iron ions and below 0.5 mg/kg for copper ions.

In the following, an embodiment of the invention is described as an example with reference to the drawings. The features of individual advantageous versions of the invention according to the above embodiments can be combined with one another as required and also left out. Furthermore, the invention is documented based on experimental examples.

In the following,

FIG. 1 shows an embodiment of a device according to the invention for the production of a cellulose solution in a schematic representation, whereby the method according to the invention can be implemented by the embodiment;

FIG. 2 shows a schematic representation of the processing steps for the production of the cellulose suspension;

FIG. 3 shows a schematic representation of the variation of the amount of the removed iron ions against time;

FIG. 4 shows a schematic representation of the chemical oxygen demand in the press water against time;

FIG. 5 shows a schematic representation of a first method for controlling the metal ion content.

FIG. 1 shows a plant 1 for the production of endless molded bodies 2, for example strands, out of a spinnable cellulose solution containing water, cellulose and tertiary amine oxide.

First, cellulose in the form of leaves or plates 3 and/or rolls 4 is fed in batches to a pulper 5. In the pulper 5, the cellulose 3, 4 is disintegrated with water as treatment medium, symbolically represented by the arrow 6, and a cellulose suspension is formed, preferably still without solvent or amine oxide. Enzymes or enzyme solutions can be added for the homogenisation and Stabilisation of the cellulose suspension.

The quantity of the added water 6 is determined in dependence of the water content of the cellulose. Typically the water content of the cellulose used is between 5 and 15 percent by mass. This span of variation is compensated by changing the addition of water appropriately, so that the water content of the cellulose suspension or the slurry ratio of solids/liquid remains approximately constant or attains a freely selected value.

From the pulper 5, the cellulose suspension is passed through a thick matter pump 7 via a pipe system 8 to a press device 9, whereby the cellulose suspension of water and cellulose is preferably maintained in a temperature range from 60 to 100° C.

In the press device, the cellulose suspension produced by the pulper 5 is expressed, for example, by rotating rolls 10. The expressed water or press water 11 is collected by a collecting device 11′ and passed back at least in part as water 6 serving as treatment medium to the pulper 5 by a conveying means 12, through an optional filter device 13 and through a mixing device 14. The press device 9 can also be provided with a suction device (not shown) for sucking off excess water from the cellulose suspension. The drawn-off water is, with this embodiment, passed back, as the press water, at least in part to the pulper 5. For the purposes of this invention, sucked-off water or water removed from the cellulose suspension by other means is also press water which can be reused for the disintegration of the cellulose.

The filter 13 can comprise one or more surface filters, deep-bed filters, membrane filters, plate filters, edge filters, separators, centrifuges, hydrocyclones, belt filters and vacuum belt filters, tube filters, filter presses, rotating filters, reversible-flow filters and multilayer filters. In addition, the press water 11 can be osmotically treated in the filter 13; alternatively or additionally, metal ions and particles can be filtered out of the press water 11 or metal-binding additives can be fed to the press water 11.

The respective proportions of the returned treatment medium 11 and of fresh treatment medium 15 fed from another fresh source, for example fresh water, in the water passed to the pulper are set by the mixing device 14. In addition, the proportion of the treatment medium 11, which is passed out of the plant 1 through a waste water pipe 16, is set by the mixing device 14.

The mixing device 14 can for example comprise a selector valve or a number of valves. The mixing device 14 is controlled by a control device 17 such that the proportions of the press water 11 and the fresh water 15 in the water 6 fed to the pulper 5 can be set to variably specifiable values by an output signal from the control device via at least one control line 18.

After expressing, the cellulose suspension is transported further through the pipe system 8 to a stirring and conveying means 19 in which a shear stress acting on the cellulose suspension is generated by a stirring or conveying tool 20, such as a screw, paddle or blade. For the stirring and conveying means 19, no annular layer mixers can be employed, such as originating from DRAIS Misch- and Reaktionssysteme and sold under the designation CoriMix®. The annular layer mixers are only used for moistening or impregnating dry cellulose materials which are not used in the method described here.

In the region of the shear stresses of the stirring and conveying means 19, in the so-called shear zone, a treatment medium such as tertiary amine oxide, in particular N-methylmorpholine-N-oxide, is passed in aqueous form via a pipe 21 to the cellulose suspension with a molar ratio NMMNO/H₂O of between 1:1 and 1:2.5 as solvent for the cellulose. In addition, in the shear zone, additives such as stabilizers and enzymes, organic additives, delustering substances, alkalis, solid or liquid alkaline earth, zeolites, finely pulverised metals such as zinc, silver, gold, platinum for the production of anti-microbial and/or electrically or thermally conducting fibers during and after the spinning process and/or dyes can be added to the cellulose suspension. The concentration of the additives can be controlled in the range from 100 to 100,000 ppm referred to the fibre product.

The concentration of the fed NMMNO depends on the water content of the celluloses 3, 4 currently in the cellulose suspension. The stirring and conveying means 19 acts as a mixer in that the tertiary amine oxide is mixed with the cellulose suspension and the cellulose solution is produced. Then, the cellulose solution to which NMMNO has been added is transported via the pipe system 8 to a second stirring and conveying means 22. The stirring and conveying means 22, can comprise a vaporization stage. From the stirring and conveying means 22 the pipe system can be heated. In contrast to the unheated pipe system 8, the heated pipe system in FIG. 1 is given the reference symbol 8′. In particular a pipe system can be used, as described in WO 01/88232 A1, WO 01/88419 A1 and WO 03/69200 A1.

After the addition of the tertiary amine oxide, the metal ion content of the cellulose solution, in particular copper and iron ions, in the pipe 8′ and/or in at least one of the shear zones 19, 22, or before and/or after one of the shear zones is measured using the sensors 23, 23′ and a signal representing the metal content or the content of individual destabilising metal ions, such as iron, chrome, copper and/or molybdenum is output to the control device 17. Alternatively or in addition to an automatic inline sampling, the metal ion content can, in a further embodiment, be determined using wet-chemical methods after manual sampling in an automatic laboratory analysis device and passed on from there to the control device 17 automatically or manually. However, with a manual sample extraction compared to the automatic inline sample extraction directly from the pipe systems 8, 8′, there is the disadvantage that the feedback to the controller for the metal ion content contains a manual process stage and cannot therefore be automated.

The control device 17 compares the metal ion content measured by the sensors 23, 23′ with predetermined limits and outputs a signal depending on this metal ion content to the mixing device 14. Due to the control signal to the mixing device 14, the composition of the water 6 passed as treatment medium to the pulper 5 is set in dependence of the content of the destabilising metal ions in the cellulose solution and the metal content or the content of individual metal ions in the cellulose solution to which tertiary amine oxide has been added is regulated under closed-loop control to a predetermined value. Since the concentration of reactions in the cellulose solution increases after the vaporization stage, preferably a sensor is provided which monitors the metal content of the cellulose solution after the addition of all ingredients and after all the vaporization stages.

If, for example, the content of destabilising metal ions in the cellulose solution, as acquired by the sensors 23, 23′ or by using wet-chemical methods, is too high, then the proportion of fresh water in the water 6 fed to the pulper 5 is increased. Thereby, the metal content is adjusted by the control device 17 such that it remains below a stability limit of 10 mg/kg. The metal content can also be determined before the formation of the cellulose solution, i.e. still in the cellulose suspension, whereby this measurement is more appropriate than the measurement of the metal content directly in the cellulose solution.

As sensors 23, 23′, devices for atomic absorption measurement, mass spectrometers, optical detectors for the acquisition of fluorescence spectra, emission spectra or Raman scattering can be used. These types of sensors are known and are produced by various manufacturers, e.g. Perkin Elmer.

During the control of the composition of the water 6, the control device 17 takes into account the previously determined metal content of the cellulose 3, 4 passed to the pulper 5. In this respect, the analyzed metal content of individual metal ions or the complete content of metal in the cellulose 3, 4 just used can be entered into the control device 17 via an input device 24. This preadjustment is taken into account in the determination of the proportions of the press water and fresh water in the water fed to the pulper 5. For example, with cellulose containing a high metal content, a higher proportion of fresh water 15 is passed to the pulper 5 at the start or certain metal-binding additives are mixed into the cellulose suspension.

If the metal content decreases, as it is acquired by the sensors 23, 23′ in the cellulose solution to which tertiary amine oxide has been added, below a certain limit which is taken as sufficient for protection against exothermic reactions, for example 10 mg/kg, then the proportion of press water in the water passed to the pulper 5 is increased. Consequently, with sufficient protection against exothermic reactions, less fresh water is consumed and less press water is discharged to the environment.

After the stirring or conveying means 22, the now extrudable cellulose solution is passed to an extrusion head 25 which is provided with a large number of extrusion openings (not shown). The highly viscous cellulose solution is extruded into an air gap 26 through each of these extrusion openings to give in each case an endless molded body 2. An orientation of the cellulose molecules occurs due to a drawing of the cellulose solution which is still viscous after the extrusion. To achieve this, the extruded cellulose solution is drawn away from the extrusion openings by a drawing device 27 with a speed which is greater than the extrusion speed.

After the air gap 26, the endless molded bodies 2 pass through a precipitation bath 28 containing a liquid such as water which is a non-solvent, whereby the cellulose in the endless molded bodies 2 is precipitated. In the air gap 26, the endless moulded bodies 2 are cooled by a cooling gas flow 29. Here, in contrast to the theory given in WO 93/19230 A1 and EP 584 318 B1, it has been found substantially more advantageous if the cooling gas flow does not emerge directly after the exit of the endless molded bodies 2 from the nozzle, but rather first at a distance from the nozzle onto the endless molded bodies 2. In order to achieve the best fibre properties, the cooling gas flow should be turbulent and exhibit a velocity component in the extrusion direction, as described in WO 03/57951 A1 and in WO 03/57952 A1.

The precipitation bath 28 becomes increasingly enriched with tertiary amine oxide so that it must be continually regenerated by means of a recovery device 30. In this respect, the liquid from the precipitation bath is fed during operation to the recovery device 30 via a pipe 31, which for example is connected to an overflow of the precipitation bath. The recovery device 30 extracts the tertiary amine oxide from the liquid and returns pure water via a pipe 32. Non-reusable waste substances are discarded from the device 1 via a pipe 33 for disposal.

In the recovery device 30, the amine oxide is separated from the water and fed via a pipe 34 to a further mixing device 35 to which fresh amine oxide is also fed by a pipe 36. The regenerated amine oxide from the pipe 34 is mixed with the fresh amine oxide 36 and fed to the shear zone 19 via the pipe 21.

Metal ions can be removed from the regenerated amine oxide via an ion exchanger, for example from Rohm and Haas, Amberlite GT 73, or via a filter 37.

The mixing device 35 and the purification device 37 can be controlled by the control device 17 in dependence of the metal ion content measured by the sensors 23, 23′.

Then, the endless moulded bodies are treated further, for example washed, brightened, chemically treated in a device 38 to influence the cross-linking properties, and/or dried and pressed out further in a device 39. The endless moulded bodies can also be processed by a cutting device, which is not shown, to form staple fibers and passed out of the device 1 in fleece form.

The overall conveyance of the cellulose solution in the pipe system 8′ occurs continuously, whereby buffer containers 40 can be provided in the pipe system 8′ to compensate variations in the conveyed amount and/or of the conveying pressure and to facilitate continuous processing without dead water regions arising. The pipe system 8′ is equipped with a heating system (not shown) to maintain the cellulose solution during conveyance at a temperature at which the viscosity is sufficiently low for an economical transport without decomposition of the tertiary amine oxide. The temperature of the cellulose solution in the pipe section 8′ is here between 75 and 110° C.

At the same time, the homogenisation and uniform mixing, which can be increased by static or rotating mixers, is promoted by the high temperature.

The residence time of the cellulose suspension or solution in the pipe system 8, 8′ from the thick matter pump 7 to the extrusion head 25 is between 5 minutes and 2 hours, preferably about 30 to 60 minutes.

The implementation of the method according to the invention is now described based on experimental examples.

In the following, values are used for the quantity which have been scaled to the amount of introduced cellulose.

A first series of experiments deals with the cellulose pretreatment for the production of the cellulose suspension and the examination of the press water. In the following, reference is made to the schematic representation of the pretreatment in FIG. 2, and furthermore the reference symbols of FIG. 1 are used.

EXPERIMENTAL EXAMPLE 1

In a process step A, cellulose 3, 4 (cf. FIG. 1) of the type MoDo Dissolving Wood Pulp, pine sulphite wood pulp, was placed in a pulper 5 from the company Grubbens with a net filling volume of 2 m³ with water 6 in a mixing ratio of 1:17 (solids density 5.5%). The cellulose exhibited a Cuoxam DP 650 and an α-cellulose content greater or less than 95%. Commercially available celluloses based on hardwood or softwood can be used. Hemicellulose contents in the cellulose in the range from 2 to 20% can also be treated in the process. Other possible celluloses are Sappi Eucalyptus, Bacell Eucalyptus, Tembec Temfilm HW, Alicell VLV and Weyerhäuser α-cellulose of less than 95%. The fed water 6 consisted of 30 parts of fully desalinated fresh water 15 and 70 parts of press water.

Under vigorous stirring, technically pure formic acid 50 in the ratio of 1:140 and a liquid enzyme preparation 51 in the ratio 1:200, referred in each case to the cellulose content, were added. An enzymatic pretreatment was then carried out for a duration of about 35 minutes until a homogeneous cellulose suspension was obtained. A cellulase enzyme complex, such as for example Cellupract® AL 70 from Biopract GmbH or Cellusoft from Novo Nordisk, can be used as the enzyme preparation 52.

Then, the pretreatment was terminated in a process step B by the addition of sodium hydroxide solution 52 in the ratio of 1:500 referred to the cellulose content of the cellulose suspension in the pulper 5.

The cellulose suspension was then dewatered in a process step C in a vacuum belt filter acting as press means 9 with following expressing system from the company Pannevis to about 50%, so that the expressed cellulose exhibited a dry content of about 50%. From step C, the expressed cellulose was then passed on via the pipe 8 for production of a cellulose solution containing NMMNO, water and cellulose. These steps are not shown in FIG. 2 for the sake of clarity.

The press water was collected in the press means 9 and let off via the pipe 11 (cf. FIG. 1). Approximately 75% of the press water was fed back to the pulper 5 and about 25% of the press water was passed via the pipe 16 to a waste water purifier.

The degree of polymerisation of the cellulose was always selected such that a DP (degree of polymerisation) of about 450 to about 550 was obtained in the spinning solution. The cellulose concentration was set to about 12% in the spinning solution.

The press water remaining in the system 1 was again mixed in a mixing device 14 (cf. FIG. 1) in a process step D with the fully desalinated water, as described above.

EXPERIMENTAL EXAMPLE 2

In another experiment all the steps of Experimental Example 1 were repeated, except that in process step A the quantity of the added enzyme preparation was reduced to 1:125 referred to the cellulose content of the cellulose suspension.

EXPERIMENTAL EXAMPLE 3

In another experiment the steps of Experimental Examples 1 and 2 were repeated, except that in process step A no enzyme preparation was added.

RESULTS OF THE EXPERIMENTAL EXAMPLES 1 TO 3

To verify the effectiveness of the method according to the invention, the press water collected during the expressing stage was analyzed for the copper and iron ion content and additionally the chemical oxygen demand was determined.

As a result of this experiment it can be recorded that, due to the circulation of a part of the press water, the acquired values of the ingredients increase in the first pulp cycles. However, since a part of the press water with the dissolved ingredients is permanently transferred out, a steady state is reached after some time in which the amount of substances contained, in particular the metal ions, remains constant.

In total about 10% of the iron ions introduced by the cellulose 3, 4 and about 40% of the copper ions introduced by the cellulose was removed by the press water feedback. In continuous plant operation, with the return of the press water, the percentage proportion of the iron extracted from the system 1 should be between 22% and 35% referred to the quantity of iron introduced by the cellulose.

FIG. 3 gives a schematic temporal progression of the iron ion extraction.

The stable final state of the system 1 is achieved, as Experimental Examples 1 to 3 show, independent of the amount of enzyme introduced for the pretreatment of the cellulose.

This is also confirmed by the temporal progression of the chemical oxygen demand (COD), as illustrated in FIG. 4. The chemical oxygen demand was determined in the press water according to DIN 38409 and approximates to a constant value with increasing duration of the press water feedback.

Furthermore, in the cellulose solutions obtained according to Experimental Examples 1 to 3, the degree of polymerisation and therefrom the DP reduction as well as the onset temperature of the spinning solution were determined as indicators of stability. The results of the experimental examples are shown in Table 1. TABLE 1 Experimental DP reduction T_(onset) Example [%] ° C. 1 27.5 165 2 27 165 3 9 160

As shown in Table 1, the cellulose solution obtained through press water feedback is stable and exhibits an onset temperature of at least 160° C. According to experiments, with this method an onset temperature of at the most 147° C. is actually obtained. The onset temperature according to Table 1 using the method according to the invention with press water feedback also lies above the onset temperature as obtained with the method of WO 95/08010 and which in practice is about 150° C.

Based on these investigations it can be seen that despite the press water feedback, the onset temperatures still lie above the onset temperatures for the dry processing of cellulose and can be increased by an enzymatic pretreatment of the cellulose. This means that the press water feedback is suitable for industrial use.

In another series of experiments, the effect of the substances contained in the press water on the stability of the cellulose solution was investigated. To achieve this, a concentrate of 5 l of press water in the ratio 1:270 was added to the cellulose solution in each of the Experimental Examples 1 and 3 and feedback of the press water was omitted.

In both cases, once according to the method of Experimental Example 1 without enzymatic pretreatment and once according to the method of Experimental Example 3 with enzymatic pretreatment, a reduction of the onset temperature to about 141° C. occurred in each case due to the press water concentrate. Thus, it is demonstrated that the press water fundamentally reduces the stability of the cellulose solution.

This destabilisation of the cellulose solution can however be prevented by discharging the treatment medium with destabilising ions. The proportion of the treatment medium fed back depends on the type of cellulose used, as shown in the following table.

The iron and copper content as well as the overall metal ion content of the cellulose varied noticeably with the various types of cellulose, as can be seen from Table 2. The metal content of the various types of cellulose was determined by incineration in the platinum crucible according to DIN EN ISO 11885 (E22) and with flame AAS. TABLE 2 Used cellulose Substances contained Cellulose Cellulose Cellulose Cellulose Cellulose Cellulose Cellulose Cellulose in cellulose 1 mg/kg 2 mg/kg 3 mg/kg 4 mg/kg 5 mg/kg 6 mg/kg 7 mg/kg 8 mg/kg Fe 1.3 2.0 1.6 5.8 2.2 2.6 14 13 Mn <0.3 <0.1 0.2 0.33 n.d. <0.3 0.4 <0.3 Mg 2 2 226 32 138 2 21 7.8 Co 0.3 <0.3 <0.3 <0.3 <0.3 <0.3 <0.3 <0.3 Ca 54 4 37 64 30 6 130 27 Cr <0.3 <0.3 1.4 <0.3 <0.3 0.4 <0.3 <0.3 Mo <0.3 <0.1 <0.1 <0.1 <0.3 <0.3 <0.3 <0.3 Ni <0.3 <0.3 <0.3 <0.3 <0.3 <0.3 <0.3 <0.3 Cu 0.3 <0.2 0.2 <0.3 <0.3 <0.3 0.3 0.3 Na 396 48 93 92 263 176 335 8.2

In a final series of experiments, the schematic experimental set-up of FIG. 5 was carried out. In FIG. 5, the reference symbols of FIGS. 1 and 2 are used for elements with similar or the same function.

With the set-up in FIG. 5, the amount of press water returned to the pulper 5 was adjusted to the iron and copper content of the expressed cellulose.

With the arrangement of FIG. 5, the iron ion and copper ion content was measured as representative values for the metal ion content by the sensors 23, 23′ (cf. FIG. 1).

Due to the control of the proportion of the press water in the water 6 fed to the pulper 5, the iron concentration was maintained as closely as possible below 10 mg/kg absolutely dry and the copper concentration just below 0.5 mg/kg absolutely dry. These values were possible for an adequate stability of the cellulose solution in the pipe 8 with simultaneous maximum retention of the press water within the system 1 and consequently with minimum outward transfer of the press water 16 from the system 1.

The control of the metal ion content occurred in such a way that if one of these two limits was exceeded, the amount of press water outwardly transferred from the system 1 and passed on for waste water purification was increased by opening a valve 58. At the same time, closure of the valve 59 reduced the proportion of press water fed back in the pretreatment stage.

If a direct pulping of the cellulose 3, 4 occurs in amine oxide, then the setting of the metal ion content according to the invention can also be achieved via the tertiary amine oxide recovered from the spinning bath 28. In this respect, the degree of purification at the metal ion filter 37 and/or the proportion of the tertiary amine oxide 36 freshly added to the regenerated tertiary amine oxide 34 can be set in dependence of the metal ion content as measured by the sensors 23 and 23′, as well as in dependence of the metal ion content previously found in the cellulose 3, 4. The control functions similarly as with the press water feedback.

In a modification of the method described in FIG. 1, water from the spinning bath 28 recovered in the recovery device 30 can be returned to the pulper 5 instead of or together with the press water.

The metal ion filter 37, as it is used for the recovery of the tertiary amine oxide from the spinning bath 28, can of course also be used for the purification of the returned press water. 

1. A method for producing a Lyocell fiber from a spinning mass containing water, cellulose and tertiary amine oxide, comprising producing the spinning mass by adding a treatment medium to a cellulose, to a cellulose suspension, cellulose solution or combination thereof, wherein the content of at least one type of destabilizing metal ions in the cellulose, cellulose suspension, cellulose solution or combination thereof is monitored and adjusted below a stability limit.
 2. The method according to claim 1, wherein the quantity of the added treatment medium is adjusted in dependence of the content of the at least one type of destabilizing metal ion.
 3. The method according to claim 1, wherein the added treatment medium is recovered in a following process step.
 4. The method according to claim 3, wherein the content of the at least one type of metal ion is reduced in the treatment medium during the recovery.
 5. The method according to claim 1, wherein a part of the treatment medium is discharged from the production process and that the quantity of the discharged treatment medium depends on the content of at least one type of destabilizing metal ion in the cellulose, cellulose suspension, cellulose solution or combination thereof.
 6. The method according to claim 1, wherein a fresh treatment medium is added simultaneously with the recovered treatment medium.
 7. The method according to claim 6, wherein the proportions of recovered and fresh treatment medium are adjusted in dependence of the content of the at least one type of metal ion.
 8. The method according to claim 1, wherein the treatment medium is dosed automatically.
 9. The method according to claim 1, wherein the treatment medium is essentially water.
 10. The method according to claim 9, wherein the recovered water is obtained by expressing the cellulose suspension.
 11. The method according to claim 9, wherein the recovered water is obtained from a spinning bath for the extruded spinning mass.
 12. The method according to claim 1, wherein the treatment medium is a tertiary amine oxide.
 13. The method according to claim 12, wherein the tertiary amine oxide is obtained from a spinning bath for the extruded spinning mass.
 14. The method according to claim 1, wherein the at least one type of destabilizing metal ion comprises iron ions.
 15. The method according to claim 1, wherein the at least one type of destabilizing metal ion comprises copper ions.
 16. The method according to claim 1, wherein the content of the at least one type of destabilizing metal ion is adjusted to below 10 mg/kg.
 17. The method according to claim 1, wherein the content of the at least one type of destabilizing metal ion is adjusted to below 0.5 mg/kg.
 18. The method according to claim 1, wherein the stability limit depends on the at least one type of destabilizing metal ion.
 19. The method according to claim 1, wherein the treatment medium is fed in a section of a shear zone.
 20. A device for production of Lyocell fibers, with comprising a mixing device, in which a cellulose, cellulose suspension, cellulose solution or combination thereof can be charged with a treatment medium, at least one sensor, by which the content of at least one type of metal ion in the cellulose, cellulose suspension, cellulose solution or combination thereof can be acquired and a control signal representing the content of the at least one type of metal ion can be output, and a control device, by which the content of the at least one type of metal ion in the cellulose, cellulose suspension, cellulose solution or combination thereof can be adjusted below a stability limit in dependence of the control signal. 